Topology evolution and gelation mechanism of agarose gel

Kinetics as well as the evolution of the agarose gel topology is discussed, and the agarose gelation mechanism is identified. Aqueous high melting (HM) agarose solution (0.5% w/v) is used as the model system. It is found that the gelation process can be clearly divided into three stages: induction stage, gelation stage, and pseudoequilibrium stage. The induction stage of the gelation mechanism is identified using an advanced rheological expansion system (ARES, Rheometric Scientific). When a quench rate as large as 30 deg C/min is applied, gelation seems to occur through a nucleation and growth mechanism with a well-defined induction time (time required for the formation of the critical nuclei which enable further growth). The relationship between the induction time and the driving force which is determined by the final setting temperature follows the 3D nucleation model. A schematic representation of the three stages of the gelation mechanism is given based on turbidity and rheological measurements. Aggregation of agarose chains is promoted in the polymer-rich phase and this effect is evident from the increasing mass/length ratio of the fiber bundles upon gelation. Continuously increasing pore size during gelation may be attributed to the coagulation of the local polymer-rich phase in order to achieve the global minimum of the free energy of the gelling system. The gel pore size determined using turbidity measurements has been verified by electrophoretic mobility measurements.     

The exterior Dirichlet problem for quasi-linear elliptic equations with small boundary data

It is shown that if the order of non-uniformity of a quasi-linear elliptic equation is h,10,2(h–1)/h norm. For 0h1,existence of a bounded solution is guaranteed without any smallness assumption on the given boundary data.More precise information is given for the special case of the minimal surface equation.     

Single-photon vacuum-ultraviolet laser-pulsed-field ionization-photoelectron studies of trans- and cis-1-bromopropenes

The vacuum-ultraviolet (VUV) pulsed-field ionization-photoelectron (VUV-PFI-PE) spectra of trans-1-bromopropene (trans-CH(3)CH[Double Bond]CHBr) and cis-1-bromopropene (cis-CH(3)CH[Double Bond]CHBr) have been measured in the energy region of 74 720-76 840 cm(-1). The simulation of fine structures observed in the origin VUV-PFI-PE vibrational bands of these molecules has provided the ionization energies (IEs) of trans-1-bromopropene and cis-1-bromopropene to be 74 779.3+/-2.0 cm(-1) (9.2715+/-0.0002 eV) and 75 140.2+/-2.0 cm(-1) (9.3162+/-0.0002 eV), respectively. The vibrational bands resolved in these VUV-PFI-PE spectra at energies 0-1700 cm(-1) above the IEs of trans-1-bromopropene and cis-1-bromopropene have been assigned based on theoretical vibrational frequencies and calculated Franck-Condon factors for the ionization transitions.     

Excess partial molar enthalpies,entropies, Gibbs energies,and volumes in aqueous dimethylsulfoxide

The excess partial molar enthalpies, the vapor pressures, and the densities of dimethylsulfoxide (DMSO)–H₂O mixtures were measured and the excess partial molar Gibbs energies and the partial molar volumes were calculated for DMSO and for H₂O. The values of the excess partial molar Gibbs energies for both DMSO and H₂O are negative over the entire composition range. The results for the water-rich region indicated that the presence of DMSO enhances the hydrogen bond network of H₂O. Unlike monohydric alcohols, however, the solute-solute interaction is repulsive in terms of the Gibbs energy. This was a result of the fact that the repulsion among solutes in terms of enthalpy surpassed the attraction in terms of entropy. The data in the DMSO-rich region suggest that DMSO molecules form clusters which protect H₂O molecules from exposure to the nonpolar alkyl groups of DMSO.     

Synthesis, structure, and nonlinear optical properties of cross-conjugated perphenylated iso-polydiacetylenes

Monodisperse, cross-conjugated perphenylated iso-polydiacetylene (iso-PDA) oligomers, ranging from monomer 15 to pentadecamer 25, have been synthesized by using a palladium-catalyzed cross-coupling protocol. Structural characteristics elucidated by X-ray crystallographic analysis demonstrate a non-planar backbone conformation for the oligomers due to the steric interactions between alkylidene phenyl groups. The electronic absorption spectra of the oligomers show a slight red-shift of the maximum absorption wavelength as the chain length increases from dimer 17 b to pentadecamer 25, a trend that has saturated by the stage of nonamer 22. Fluorescence spectroscopy confirms that the pendent phenyl groups present on the oligomer framework enhance emission, and the relative emission intensity consistently increases as a function of chain length n. The molecular third-order nonlinearities, gamma, for this oligomer series have been measured via differential optical Kerr effect (DOKE) detection and show a superlinear increase as a function of the oligomer chain length n. Molecular modeling and spectroscopic studies suggest that iso-PDA oligomers (n>7) adopt a coiled, helical conformation in solution.     

Synthesis of amino acid-containing polyacids and their application in self-cured glass-ionomer cement

Six methacrylate or acrylate derivatives of natural amino acids were synthesized and characterized. Based upon these monomers, six terpolymers [poly(acrylic) acid-co-itaconic acid-co-amino acid] were prepared and characterized. The synthesized polymers were used to formulate glass-ionomer cements (GICs) using Fuji II glass filler. The effects of the molecular weight (MW) and powder/liquid (P/L) ratio were evaluated. Scanning electron microscopy (SEM) was used to examine the fracture surfaces of the selected cement specimens. Results show that all the amino acid modified GICs exhibited higher compressive strengths (CS, 193-236 MPa) and much higher flexural strengths (FS, 55-71 MPa) as compared to commercial Fuji II GIC (191 in CS and 16 in FS). Both MW and P/L ratio affected the strength of the formed cement. It was important to find the optimal MW and P/L ratio to obtain the highest FS. In this study, optimized MW (number average) of the polyacids and P/L ratio were around 50,000 and 2.7/1, respectively. The microstructures of the fracture surfaces helped to explain the strength differences among the materials tested in the study. SEM analysis suggests that more integrated microstructures and fewer defects can lead to higher FS.     

Pore assembled multilayers of charged polypeptides in microporous membranes for ion separation

In this study, highly permeable ion-selective membranes are prepared via immobilization of polyelectrolyte multilayer networks within the inner pore structure of a microporous (pore size = 0.2 microm) support. Electrostatic layer-by-layer assembly is achieved through alternate adsorption of cationic and anionic polyelectrolytes under convective flow conditions. To initiate pore assembly, the first layer consists of covalently bound charged polypeptides (poly(L-glutamic acid) (PLGA) or poly(L-lysine) (PLL)) establishing a charged support for subsequent adsorption. Nonstoichiometric immobilization of charged multilayers within a confined pore geometry leads to an enhanced volume density of ionizable groups in the membrane phase. This overall increase in the effective charge density allows for Donnan exclusion of ionic species (especially divalent co-ions) using microporous materials characterized by permeability values that exceed conventional membrane processes. Multilayer assemblies are fabricated using both PLGA/PLL and synthetic polyelectrolytes (poly(styrenesulfonate)/poly(allylamine)) in an attempt to compare the level of adsorption and separation properties of the resulting materials. The role of salt concentration in the carrier solvent on overall polyelectrolyte adsorption was examined to determine its effect on both solute (Cl-, SO4(2-), As(V)) and water transport. Constriction of the pore size induced by multilayer propagation was monitored through permeability measurements and dextran rejection studies at each stage of the deposition process.